The invention relates to printing, and more specifically to flexographic printing and in particular to the fabrication of flexographic printing plates.
In flexographic printing most plates are prepared by using a photo-polymer exposed through a mask. After exposure many steps are required to complete the preparation of a flexographic plate. For example, the unexposed polymer is washed away by brushing with a solvent, followed by extended drying. Another method of making flexographic plates uses direct ablation of the polymer with a high power CO2 laser. This method is slow due to the large amount of material to be removed (about 1 kg of material removed from 1 m2 of plate). There is therefore a need for lower cost, higher speed methods for the preparation of flexographic printing plates.
A range of technologies is now available to create three-dimensional structures directly from digital information. These techniques are most typically applied in the design-engineering field to create three-dimensional models as a part of the mechanical design process. Many of these techniques are very sophisticated such as stereo-lithography using lasers to expose a photo-sensitive resin which then hardens. One particular technique that lends itself to cost-effective fabrication of three-dimensional structures is a modified inkjet process.
Standard ink-jet processes typically employ water-based inks, although some systems operate using oil-based inks. Some ink-jet devices are of the type known as xe2x80x9csolid ink jetxe2x80x9d or xe2x80x9cphase-change ink jetxe2x80x9d. In those types of ink jets the ink is solid at room temperature, becomes a liquid by heating it in the ink-jet head, and solidifies again when cooling down after being deposited on the printed substrate. The best known commercial example of xe2x80x9csolid inkxe2x80x9d jet is the product line made by Tektronics Inc. (Beaverton, Oreg.) and sold under the generic name xe2x80x9cPhaserxe2x80x9d. These units use a multi-channel heated head to deposit droplets of molten wax on paper. Solid ink ink-jet deposition is not limited to planar objects. Small three-dimensional objects can be built-up by depositing many layers of molten polymer or wax. A commercial unit for building such small models is sold by 3D Systems (Valencia, Calif.) under the name Actua 2100. The brochure describing this system is hereby made of record.
The waxes and polymers used by the Tektronics units or the 3D Systems units are not suitable for flexographic plates. However, both the Tektronics Phaser and the 3D Systems Actua 2100 are very suitable for deposition of liquid elastomers as long as the elastomer reaches a sufficiently low viscosity at the working temperature of these units (100xc2x0 C. to 150xc2x0 C. range). Alternatively, a liquid may be ink-jetted, followed by post-curing. A well-known method of post-curing is the use of UV light to solidify inks based on photo-polymers.
A flexographic plate resembles a very large rubber stamp. A polyester back, typically 0.007xe2x80x3 (0.17 mm) thick provides mechanical stability and an elastomeric raised image, having a typical relief of 0.006xe2x80x3 to 0.04xe2x80x3 (0.15 to 1 mm) provides the printing surface. Since each path of the Actua 2100 system builds up 0.001xe2x80x3 to 0.004xe2x80x3 of thickness, between 2 and 40 passes are required to build up the relief required for printing.
There is, however, a debilitating problem in applying elastomer-based inkjet techniques to the fabrication of flexographic printing plates. The flexographic printing process is remarkably sensitive to the detailed surface finish of the raised areas of the flexographic printing plate. Typical nominally flat surfaces grown by elastomeric ink-jet deposition do not meet the stringent surface finish requirements of the printing industry. In order for the economic benefits of inkjet deposition to be brought to bear on the printing industry, it is therefore necessary to make some critical adaptations to the established methods used with this technology.
It is an object of this invention to devise an inkjet-based plate-making process in which the quality of the printing surface is much higher than that achievable by a simple xe2x80x9cbuild-upxe2x80x9d of layers. Another object is to make the quality of the printing surface independent of deposition conditions.
In accordance with the present invention an elastomeric printing plate, with a printing surface finish that allows it to be used in flexographic printing, is grown by a method based on an inkjet process. By employing two different materials, one used as a removable filler, the printing plate is grown in inverted orientation with its printing surface against a modifying surface that modifies the growth of the printing plate and, in particular, ensures that the raised image on the printing plate has a smooth surface finish. The present invention allows the generation of a flexographic printing plate in a short time without the need for high power CO2 lasers and liquid processing steps.